Kyle every situation is a bit different, so without seeing why you couldn't drag that calc onto the filter shelf I'd just be guessing. I'm attaching a workbook using Superstore data showing the calc works. If you can post back with a calc like this that doesn't work, I'll take a look and see why it's not working.

I'm having a hard time recreating it and would have to dumb out the data to share my actual workbook, but I think I can narrow down the why. Also, I've got the solution I want working so if this doesn't make sense no worries.

Couple things right off, I'm using two sources that are exactly identical but from a 30 day difference as a snapshot in time and my =T/F is off the results of another caluclation using both sources. So....

I am fond of the logical NOT operator for things like this, that is, we make a condition that includes everything we don't want, and then put a parenthesis around it and write NOT in front of it to exclude it.

You'll note that my original calculation was NOT aggregated. Whereas your calculation by definition (coming from a secondary data source) MUST be aggregated. So it's not that you are using a secondary data source it is that your calculation is aggregated. If I had written SUM([Measure])< -1 OR SUM([Measure]) > 1 then Tableau would have automatically created a T/F in the measures window, and would not have allowed me to drag it to the dimensions window. When I created the non-aggregated calc, T automatically put it into the dimensions window, and T won't let me drag it to the measures window. So you're half-right.